全文获取类型
收费全文 | 8554篇 |
免费 | 348篇 |
国内免费 | 86篇 |
专业分类
化学 | 5274篇 |
晶体学 | 83篇 |
力学 | 333篇 |
数学 | 1544篇 |
物理学 | 1754篇 |
出版年
2023年 | 53篇 |
2022年 | 130篇 |
2021年 | 197篇 |
2020年 | 200篇 |
2019年 | 215篇 |
2018年 | 206篇 |
2017年 | 187篇 |
2016年 | 298篇 |
2015年 | 233篇 |
2014年 | 349篇 |
2013年 | 587篇 |
2012年 | 556篇 |
2011年 | 613篇 |
2010年 | 359篇 |
2009年 | 399篇 |
2008年 | 480篇 |
2007年 | 409篇 |
2006年 | 348篇 |
2005年 | 307篇 |
2004年 | 303篇 |
2003年 | 254篇 |
2002年 | 258篇 |
2001年 | 182篇 |
2000年 | 171篇 |
1999年 | 102篇 |
1998年 | 87篇 |
1997年 | 90篇 |
1996年 | 116篇 |
1995年 | 93篇 |
1994年 | 78篇 |
1993年 | 86篇 |
1992年 | 65篇 |
1991年 | 64篇 |
1990年 | 57篇 |
1989年 | 56篇 |
1988年 | 45篇 |
1987年 | 62篇 |
1986年 | 65篇 |
1985年 | 69篇 |
1984年 | 57篇 |
1983年 | 54篇 |
1982年 | 52篇 |
1981年 | 50篇 |
1980年 | 35篇 |
1979年 | 49篇 |
1978年 | 43篇 |
1977年 | 38篇 |
1976年 | 44篇 |
1975年 | 37篇 |
1974年 | 28篇 |
排序方式: 共有8988条查询结果,搜索用时 15 毫秒
131.
Dense membranes were prepared from poly(vinyl alcohol)–poly(N-vinyl pyrrolidone) (PVA–PVP) blends of different compositions and studied in swelling and dehydration by pervaporation of three organic solvents contaminated by 5 wt% water. The swelling generally increases with the PVP content. No extraction occurs in water–tetrahydrofuran (THF) and water–methyl ethyl ketone (MEK) mixtures. In ethanol containing 10 wt% of water, there is no extraction for blends containing less than 40 wt% PVP and an increasing extraction beyond this PVP content. The pervaporation flux of the water–ethanol mixture increases drastically at the same threshold whereas the water permselectivity falls to a low level. The values of the diffusion and permeability coefficients determined from transient permeation of the test water–ethanol mixture exhibit a similar sudden increase at the same PVP content threshold. This singular behavior of the blend membranes is interpreted by a strong affinity of the PVP component to ethanol, combined with a disappearance of crystallites in the blend at this threshold. Consequently the amorphous membrane can swell freely according to the affinity of the PVP component, leading to the observed behavior. 相似文献
132.
Nguyen Thoai 《Journal of fluorine chemistry》1975,5(2):115-125
Perfluoroalkylmagnesium bromide, CnF2n+1MgBr (n = 2, 6, 8), reacts with α,β-unsaturated aldehydes to give allylic alcohols as the sole products. No attack on the β-carbon atom of the carbonyl was detected, even when the reaction was carried out in the presence of CuCl. Part of the Grignard reagent decomposes to trans-RFCFCFBr and trans-RFCFCFI. Neither a mechanism involving dissociation into R+F MgX- nor the formation of a carbene intermediate is capable of explaining the thermal decomposition of the Grignard reagent. 相似文献
133.
Yong-Sam Chung Sun-Ha Kim Moon Jong-Hwa Young-Jin Kim Jong-Myoung Lim Jin-Hong Lee 《Journal of Radioanalytical and Nuclear Chemistry》2005,267(1):95-107
Summary As part of an air pollution monitoring study, airborne particulate matter (PM2/PM10-2) samples were collected from 2000 to 2003 at two sampling sites in an urban region, Daejeon, the middle of Korea. Mass concentrations
of both fine and coarse fractions and that of the black carbon in the fine particles were measured using the Gent stacked
filter unit sampler and the smoke stain reflectometer, respectively. In the collected samples the concentrations of 24 elements
were analyzed using instrumental neutron activation analysis. Monitored data were investigated for their temporal trends under
different environmental conditions and their seasonal correlation patterns. Crustal enrichment factors were also estimated
to establish the contribution between anthropogenic and crustal origin. Patterns for airborne particle matter (APM) and elemental
concentrations, seasonal variation of some marker elements were investigated. The results can be applied for the investigation
of further air pollution sources and for the evaluation of air quality. 相似文献
134.
Existing theories of the motion of foam films in capillaries often assimilate the pressure drop over the foam films to the static capillary pressure obtained from the Young-Laplace equation. Hence, they ignore the contribution of dynamic effects associated with the rapid stretching and contraction of the foam films to the overall viscous dissipation. This paper reports an investigation of the motion of foam films in axisymmetric diverging-converging channels, taking into account surface viscosity and elasticity. First, a phenomenological theory for the motion of the foam films is developed using simple physical arguments. We show that the displacement of the film obeys a nonlinear second-order differential equation, which can be solved numerically for the (dimensionless) distance from the inlet and the pressure drop as a function of time. Experiments with foam film motion, conducted using glass diverging-converging channels (minimum radius = 3.00 +/- 0,01 mm, maximum diameter = 7,98 +/- 0,01 mm) and nitrogen foam stabilized with sodium dodecyl sulfate (SDS) in brine, are discussed. For a single film motion in the diverging channel, we find that (a) the static pressure drop is a concave-upward function of distance and decreases from 1.0 to about 0.3, whereas (b) the dynamic pressure drop is concave downward and increases from 1 to a maximum of 1.3 and then decreases to 0.7. In the converging channel both the static and dynamic pressure drops are concave-downward functions, but the dynamic pressure drop values are always higher than the static ones. For two films the motions were found to be rather sensitive to the initial arrangement in the channel. The experiments are found to be in excellent agreement with the theoretical predictions. These observations imply that the large flow resistance obtained during foam flow in granular porous media, where converging-diverging channels are abundant, is largely due to the surface elasticity and viscosity of the films. 相似文献
135.
A model system for the synthesis of phloroglucinol containing natural products was synthesized. Key steps include a manganic acetate-mediated cyclization and the facile conversion of an alkene into a β-bromoenone. 相似文献
136.
Yong-Sam Chung Jong-Hwa Moon Kwang-Won Park Sun-Ha Kim Jin-Hong Lee Kil-Yong Lee 《Journal of Radioanalytical and Nuclear Chemistry》2002,254(1):117-128
Instrumental neutron activation analysis was used for the analysis of 25 trace elements in airborne particulate matter (PM) for air pollution monitoring. For the collection of air samples, the Gent stacked filter unit low volume sampler and two types of Nuclepore polycarbonate filters were employed. Samples were collected at selected sampling dates in suburban and industrial regions of Daejon city in the Republic of Korea. Mass concentrations and black carbon of PM were measured, and enrichment factors were calculated. The results were used to describe the emission sources and their correlation patterns. 相似文献
137.
138.
Yasuo Saegusa Sakayu Iida Shigeo Nakamura Nguyen Chau Yoshio Iwakura 《Journal of polymer science. Part A, Polymer chemistry》1984,22(5):1017-1023
Aromatic polythioamide-oxothioxoquinazolines were synthesized by the polycondensation of 2,2′-(m-phenylene)bis-1,3,4-thiadiazoline-5-thione with aromatic bis-o-amino esters. The polymerizations were carried out at 160°C in acidic media such as m-cresol, sulfolane, and polyphosphoric acid to produce polymers with reduced viscosities up to 0.5 dL/g. These polymers were soluble in polar aprotic solvents like N-methyl-2-pyrrolidone and some acidic media including m-cresol. The polythioamide-oxothioxoquinazolines showed relatively good thermal stability with 10% weight loss at 344–394°C in air. 相似文献
139.
Simultaneous measurement of glucose and glutamine in aqueous solutions by near infrared spectroscopy 总被引:1,自引:0,他引:1
Hoeil Chung Mark A. Arnold Martin Rhiel David W. Murhammer 《Applied biochemistry and biotechnology》1995,50(2):109-125
A method is described for measuring the concentrations of both glucose and glutamine in binary mixtures from near infrared
(NIR) absorption spectra. Spectra are collected over the range from 5000–4000/cm (2.0–2.5μm) with a 1-mm optical path length.
Glucose absorbance features at 4710, 4400, and 4300/cm and glutamine features at 4700, 4580, and 4390/cm provide the analytical
information required for the measurement. Multivariate calibration models are generated by using partial least squares (PLS)
regression alone and PLS regression combined with a preprocessing digital Fourier filtering step. The ideal number of PLS
factors and spectral range are identified separately for each analyte. In addition, the optimum Fourier filter parameters
are established for both compounds. The best overall analytical performance is obtained by combining Fourier filtering and
PLS regression. Glucose measurements are established over the concentration range from 1.66–59.91 mM, with a standard error of prediction (SEP) of 0.32 mM and a mean percent error of 1.84%. Glutamine can be measured over the
concentration range from 1.10–30.65 mM with a SEP of 0.75 mM and a mean percent error of 6.67%. These results demonstrate
the analytical utility of NIR spectroscopy for monitoring glucose and glutamine levels in mammalian and insect cell cultures. 相似文献
140.
A novel photoinduced ring opening and isomerization of adamantane-2-spiro isoxazolines using Mo(CO)6
The Mo(CO)6-mediated photoinduced ring-opening reactions of adamantane isoxazolines involve novel rearrangement that provide enaminoketones as major products and β-hydroxy ketones as minor ones; in contrast, only β-hydroxy ketones and α,β-unsaturated ketones were obtained under thermal condition. 相似文献